This invention relates to methods of and apparatus for connecting optical fibers. In particular, it relates to lenses to couple light from one fiber into another, and to the design of such lenses so as to permit improved packaging of optical fibers, lenses, and related components.
In optical fiber applications, it is often necessary to couple light from one fiber into another. This might be done at a switching facility where multiple fibers are brought together. A known way to do this is by directly butting the fibers together. The fibers can also be joined by electrical fusion. In this method, an electric arc is used to heat the end of the two fibers as they are brought into contact. The arc melts the fibers, causing them to join in a permanent and mechanically stable joint. It is also possible to use lenses to couple the light from one fiber into another, as described in U.S. Pat. No. 4,421,383, which describes lenses and a physical connector that holds the fiber and lenses in appropriate positions.
In many applications, it is desirable to perform processing or manipulation of the light after the light exits the source fiber and before it enters the receiving fiber. Examples of this processing include attenuation and filtering. In optical communication systems that utilize multiple wavelengths on one fiber, referred to as wavelength division multiplexing, an erbium-doped fiber amplifier is used to optically amplify the optical signal in the fiber over a broad wavelength range. Since each wavelength in a wavelength division multiplexed system comes from a different source, the signal power at each wavelength may need to be adjusted for optimum operation of the optical amplifier. The adjustment of the signal power requires variable optical attenuation of the optical signal, and this attenuation is often most easily performed on an expanded beam.
Processing of the optical signal between fibers is most easily performed if the optical beam from the fiber has been expanded and collimated. FIG. 1 shows an example of a pair of conventional collimating lenses 10 and 12 being used to couple light from a source fiber 14 into a receiving fiber 16. It is known in the art that gradient index lenses are commonly used for this application. Gradient index lenses are made by diffusing a dopant into a cylindrical glass body. The dopant produces a radial gradient in the refractive index of the lens. If the refractive index is less towards the periphery of the lens, then the lens will focus light from a distant source. The shape of the refractive index profile controls the imaging properties of the lens. After diffusion, the lenses are cut to a specific length and the ends are polished. When the light is collimated between the lenses, the beam stays nearly the same size over an appreciable distance xe2x80x9cDxe2x80x9d (typically 10""s of millimeters). Since the beam is nearly the same size in this space, it is easier to put additional optical components that either attenuate or filter the beam, such as for example optical modulator xe2x80x9cMxe2x80x9d shown in FIG. 2.
In systems involving the processing of optical signals, it is desirable to maintain as much signal power as possible when coupling the optical signal from one fiber to another. For the case of single mode optical fiber, the coupling efficiency can be computed by analytical methods. (See R. E. Wagner and J. Tomlinson, xe2x80x9cCoupling efficiency of optics in single-mode fiber components,xe2x80x9d Applied Optics, vol. 21, No. 15, 1982, pg 2671). For the case of coupling light from one fiber to the other, the lenses must be of a specific optical function in order to produce high coupling efficiency. Second collimating lens 12 produces a focused beam that is directed towards receiving fiber 16. The percentage of light coupled into the receiving fiber will be reduced by any aberrations in the focused beam. Loss of optical power in a fiber system is highly undesirable, as it can limit the amount of information that can be transferred over a communication channel.
Recently, more optical fiber based communication systems utilize multiple wavelengths at one time in order to increase the quantity of information carried. The general concept of using multiple wavelengths is referred to as wavelength division multiplexing. Wavelength division multiplexing systems require a method to separate out signals of different wavelengths present in the optical fiber. This can be done by a method as shown in FIG. 3. A source fiber 18 is located near the front focal plane of a collimating lens 20. Light from the source fiber is collimated by collimating lens 20 and directed at an optical filter 22. A coating of the optical filter is constructed to reflect all light except that light in a very narrow wavelength band centered around a desired wavelength. Light that passes through filter 22 is coupled into a receiving fiber 24. If filter 22 is aligned correctly, light reflected from the filter will be directed onto the end of a second receiving fiber 26. Note that fibers 18, 24, and 26 are located off the optical axis of the system.
To achieve high coupling efficiency of a beam into an optical fiber, it is not sufficient that the beam be focused onto the fiber with a low amount of aberration. More specifically, the focused beam must match the fundamental mode of the fiber. This requires that the beam be of the same amplitude and phase of the fiber mode. To match the phase distribution of the fiber, the beam should enter the fiber along the optical axis of the fiber, or additional loss will result. If we assume that the end face of the fiber is perpendicular to the optical axis of the fiber, then the beam must be perpendicular to the fiber for the highest coupling efficiency. For a normal imaging system, the condition of a beam being parallel to an axis of the system is referred to as telecentricity. More specifically, telecentricity in a normal imaging system requires that the chief ray, which is the ray traveling through the center of the stop, be parallel to the optical axis at some point in the system. An optical system may be telecentric at different portions of the optical system. If the chief ray is parallel to the optical axis in object space, one would consider the system to be telecentric in object space. If the chief ray is parallel to the optical axis in image space, one would consider the system to be telecentric in image space. For example, FIG. 4 shows a simplified system of a lens 28 and a stop 30 wherein the system is telecentric in object space. FIG. 5 shows a similar system of a lens 28xe2x80x2 and a stop 30xe2x80x2 that is telecentric in image space.
Due to the nature of the fiber source, the beam coming from an optical fiber would normally be considered to be telecentric in object space, as that beam emerges from the fiber parallel to the optical axis. It is a desirable feature of the optical system for coupling fibers that the light is also telecentric in image space, in order to achieve the highest coupling efficiency of light into the receiving fiber, which is located in image space. If the light enters the optical fiber at a substantial angle to the axis of the optical fiber, then the coupling efficiency of the beam into the fiber will be significantly reduced. Although it may be possible to tilt fibers from the optical axis in order to reduce the effective angle between a beam and the optical axis of the fiber, tilting fibers can greatly increase the time and cost of assembling the final optical system. The conditions of telecentricity are affected by the location and type of the optical elements, and the location of the stop.
For systems used to couple light from one fiber to another, it is not desirable to have any apertures that limit the beam and thereby reduce optical power. Hence there is often no defined aperture or stop limiting the beam. When there is no physical aperture limiting the beam, telecentricity is determined by the characteristics of the source and receivers in combination with the optical elements. More specifically, if a beam is propagating in the system and it is undesirable to introduce any aperture that would limit the optical beam in any way, then the location of the stop is usually described by the location of where the chief ray crosses the optical axis of the system. The chief ray is defined to be the ray in the center of the beam distribution that is emitted from the source, and hence is not determined by physical apertures in the optical system.
It is known in the art that gradient index lenses can be used to collimate light from optical fibers. Such lenses are made by Nippon Sheet Glass, Somerset, N.J. When a gradient index lens is of a length such that light emitted from a point source on one face is collimated, then the lens is referred to as a quarter-pitch lens.
A significant disadvantage of gradient index lenses is shown in FIG. 6. Here a quarter-pitch rod lens 32 is shown collimating the light from a fiber 34. If the fiber is placed off the optical axis 36 of the system, as shown in the figure, then the chief ray 38 (represented by the ray that is emitted perpendicularly to the fiber face) crosses the optical axis at the exit face 40 of quarter pitch rod lens 32. In order to focus this beam on a fiber and produce high coupling efficiency, a second quarter pitch rod lens 42 must be placed in close proximity to exit face 40 of first quarter pitch rod lens 32. In this way, the chief ray emerging from second quarter pitch rod lens 42 will be nearly parallel to the optical axis of the lens and of a receiving fiber 44 after emerging from the second lens 42, thus ensuring high coupling efficiency. If the second quarter pitch rod lens is displaced longitudinally from the first quarter pitch rod lens, as shown in FIG. 7, the chief ray will not be parallel to the axis of the receiving fiber after the chief ray emerges from the lens.
If gradient index lenses are used for the lenses in FIG. 3, the filter requires that the lenses be separated. This in turn means that the chief ray will not emerge from the receiving lens parallel to the optical axis, or in terms previously described, the system is not telecentric in image space. The lack of telecentricity means that the optical system will have higher loss, unless the fibers are realigned to account for the lack of telecentricity.
An example of a commercially available gradient index lens is SLW18 by Nippon Sheet Glass. FIG. 8 shows the theoretical coupling efficiency as a function of lens separation (D) for the NSG SLW lens pair. This example assumes that the fibers are spaced 100 microns from the optical axis, and operating at a wavelength of 1.56 microns. Note that the off-axis coupling efficiency drops dramatically as the separation between the two lenses increases.
In order to have high coupling efficiency, the focusing lens must not introduce significant aberrations into the beam. For a gradient index lens, the shape of the refractive index profile must be tailored exactly to produce minimal aberrations. The control of the refractive index profile is difficult, since the shape of the profile is controlled by only by diffusion of the dopant into the glass.
It is a further disadvantage of gradient index lenses that one of the dopants commonly used in the diffusion is thallium. For example, the use of thallium in gradient index lenses is described in U.S. Pat. Nos. 3,941,474 and 4,246,016. Thallium is a toxic metal (even more toxic than lead).
In addition to gradient index glass lenses, previous workers have used refractive lenses to couple light between fibers, as mentioned in U.S. Pat. 4,421,383. However, U.S. Pat. No. 4,421,383 does not describe the use of aspheric surfaces to improve optical performance, and does not discuss the conditions needed to improve off-axis coupling efficiency between off-axis fibers. U.S. Pat. No. 5,301,249 (Hamblen et al) describes the use of mirrored systems to couple light from a laser diode into a fiber. However, this patent does not quantitatively describe expected single-mode coupling efficiencies, nor does it describe off-axis performance of the system
It is an objective of the present invention to provide a prescription for lenses that produce high coupling efficiency between optical fibers.
It is another object of the present invention to allow high efficiency of coupling light from one fiber to a second single mode receiving fiber.
It is still another object of the present invention to allow high coupling efficiency between single mode fibers even when the optical fibers are located away from the optical axis of the lens.
According to a feature of the present invention, the lens uses a refractive molded convex shape on one surface and a flat surface on the other. All of the optical power of the lens is concentrated on the one convex surface. As discussed below, this has significant advantages when the lenses are used in pairs to couple light between fibers.
In another feature of the invention, the convex surfaces can be aspheric surfaces, where the amount of asphericity is controlled in order to minimize aberrations for beams passing through the lens.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.